Heretofore, gas sensors have been developed which are attached to an internal combustion, like an automobile engine, and detect specific gas components in exhaust gas (a gas to be measured). Among such gas sensors, a gas sensor (oxygen sensor), which detects oxygen concentration, and a NOx sensor, which detects nitric oxide gas concentration, are known to use a gas sensing element composed of a solid electrolyte such as zirconia.
Generally, the gas sensors of this type have a structure in which a gas contact part of the gas sensing element is exposed to the exhaust gas. The gas sensing element is heated to high temperatures (approximately 300° C.) by a heater to be activated, and detect the specific gas components in the exhaust gas.
The gas sensing element is made of ceramics and hence, sensitive to thermal shock. Therefore, if moisture in the exhaust gas is adhered to the gas sensing element which has been heated to high temperatures, a crack may be produced and cause damage to the gas sensing element.
For the above reason, a protector that covers the gas contact part of the gas sensing element is attached to the gas sensor so as to protect the gas sensing element from water droplets.
The protector is provided with inlet openings and outlet openings for the gas to be measured in its side wall or bottom wall. The gas to be measured is introduced from the inlet openings of the protector to the gas contact part of the gas sensing element and discharged from the outlet openings. The protector performs the introduction and discharging of the gas to be measured.
There is a gas sensor comprising a two-tiered protector composed of an inner hollow-cylindrical portion (a first hollow-cylindrical portion) and an outer hollow-cylindrical portion (a second hollow-cylindrical portion), in order to effectively remove moisture contained in the gas to be measured, and introduce and discharge the gas to be measured.
In this gas sensor, a side wall of the inner hollow-cylindrical portion and a side wall of the outer hollow-cylindrical portion are coaxially disposed with an air space therebetween. In these side walls, inlet openings of the gas to be measured (first gas inlet openings and second gas inlet openings) are formed. Also, at the inlet openings of the outer hollow-cylindrical portion, guiding bodies are disposed to produce a swirling flow that surrounds an outer face of the side wall of the inner hollow-cylindrical portion. Thereby, the gas to be measured introduced from the inlet openings of the outer hollow-cylindrical portion is divided into relatively heavier water droplets and relatively lighter gas components by a function of the guiding bodies for producing the swirling flow. The gas to be measured from which the moisture is removed is carried into the inner hollow-cylindrical portion from the inlet openings of the inner hollow-cylindrical portion, and brought into contact with the gas sensing element so that the specific gas components in the gas to be measured are detected. After this, the gas to be measured is passed through a discharge opening (a first side gas discharge opening) provided in the bottom wall of the inner hollow-cylindrical portion and discharged from a discharge opening (a second side gas discharge opening) provided in the bottom wall of the outer hollow-cylindrical portion (e.g., refer to a patent literature 1).
[Patent literature 1]
Unexamined Japanese Patent Publication No. 2001-099807 (pages 4 to 7, FIGS. 1 to 5)